Today, chains in countless variants are used in the construction of machines and systems as, for example, drive chains of continuous transporters for the transportation of persons, in particular of escalators, conveyors, and moving walks.
Driving elements drive the chain or step chain or pallet chain in the direction of circulation, while by means of rotation reversing elements transfer their individual translatory belt segments into each other. Preferably, but not necessarily, driving elements and reversing elements coincide and are executed in the form of, for example, chain wheels or wedge disks. Accordingly, now follows a short discussion of such engagement elements that engage with the chain or step chain by positive and/or non-positive engagement with the chain or step chain which they drive and/or reverse.
The engagement elements cause fluctuations in the speed of the chain strand in the longitudinal direction (i.e. in the direction of movement of the chain) and in the normal transverse direction thereto as a result of the so-called polygon effect. This results from the reversal of the individual chain links when running onto the chain wheel or engagement element. When this happens, the chain links experience a sudden acceleration perpendicular to the direction of circulation of the chain strand, because the individual chain links suffer a sudden rotational impulse—a running-in jerk or running-in thrust. Conversely, on running out, this rotational impulse causes the chain to roll in in the direction of rotation of the engagement element.
For a fuller understanding of the polygon effect, which as a result of induced vibrations is the main source of noise generation on maintained chains, causes them to wear, and, on people transporters, is experienced by the passengers as an unpleasant irregularity of motion, reference should be made to the relevant specialized literature, for example P. Fritz: Dynamik schnelllaufender Kettentriebe, VDI-Verlag, 1998, to which reference in its entirety is made.
With a conventional engagement element 100, illustrated diagrammatically in FIG. 1, a chain 200 runs into the pitch circle 500 tangentially in such manner that the chain pins 300 subsequently run on the pitch circle 500 with radius R500. When, as shown in idealized form in FIG. 1, a pin in a vertical plane (shown dotted) enters for the first time into engagement with the element 100, from that point on the pin is forced to travel with a velocity v=R500×ω, where ω is the constant speed of rotation of the engagement element. Its velocity L=v×cos(α) in the longitudinal direction of the loaded end (in the drawn plane of FIG. 1, horizontal) reduces as angle α increases. Correspondingly, the loaded end is moved with this reducing horizontal speed L until the next pin 300 enters into engagement with element 100 and is suddenly accelerated to v. This results in the periodically fluctuating end velocity L=R500×ω×cos (α).
To avoid the polygon effect, WO 00/07924 proposes, as shown diagrammatically in FIG. 2, to gradually transfer the chain pins 310 from a smaller active circle (shown vertically dotted in FIG. 2), onto which the chain 210 runs tangentially, over a partially curved guide rail (not shown) onto the larger pitch circle 510 (shown dotted at angle α in FIG. 2). Simplified, should the radius r, on which the running-in chain pin 310 is guided, increase in the ratio r (α)=R500/cos (α), a constant end velocity L=R500×ω can be generated, while the velocity of the chain pin w increases correspondingly to w=R510×ω.
The engagement element is executed as a chain wheel 110 with constant pitch circle 510. It can be regarded as disadvantageous that the chain rollers in the area of the curved guiderails are lifted off the tooth bases of the chain wheel, i.e. they drift on the pitch circle relative to the engagement element, which causes generation of noise as well as premature wear. Shown by way of explanation in FIG. 2 is the engagement situation in which the chain pin 310 runs onto the tooth base at its lowest point. In this simplified illustration, the earlier start of engagement resulting from real contact geometry is ignored without the basic principles being affected. As can be seen by reference to the tooth spaces in the left part of the drawing, the chain pin 310 passes from the smaller active circle to the larger pitch circle 510 and thereby slides upwards within the tooth space relative to the teeth of the chain wheel 110.